The present invention relates to a zero-phase current transformer (hereinafter referred merely to ZCT when applicable) for use as a leakage current detecting element in a leakage current interrupter.
In FIG. 1 is a schematic diagram including a partially cross-sectional view illustrating an example of conventional ZCT. In FIG. 1, reference numeral 1 designates a circular magnetic core made of magnetic material; reference numeral 2, a damper member such as silicon grease for protecting the magnetic core 1 from impact directly applied thereto; reference numeral 3, a protective case for protecting the magnetic core 1 from an inward pressure caused by a secondary winding 4 wound around the peripheral surface thereof; and reference numeral 5 designates the ZCT generally including the above described components. After providing the secondary winding 4 over the protective case, either a winding of a tape with lead lines or an additional protective case is further provided around the secondary winding 4 to provide further protection to the winding 4. As the present invention specifically relates to the construction of the magnetic core 1 in the ZCT 5, a description and illustration of the protective means for the secondary winding 4 will be omitted.
In the conventional ZCT, magnetic materials such as Supermalloy (Trademark, the material itself is defined by JIS C 2531) having a high magnetic permeability are used as the magnetic core 1. The magnetic core 1 may be constructed by either, as shown in FIG. 1, piling annular magnetic core materials shaped from discs by piercing or, as shown in FIG. 2, spirally winding a ribbon-shaped magnetic core material.
FIGS. 3A and 3B are explanatory diagrams for describing an operation principle. FIG. 3A shows a circuit diagram of a commonly-used circuit employing such a transformer while FIG. 3B shows an equivalent circuit thereof. In FIG. 3A and FIG. 3B, reference numeral 10 designates a load lead line. Two load lead lines in the case of single phase current or three load lead lines in the case of three phase current extend through the center of the ZCT 5 or are wound around the ZCT 5. Reference numeral 11 designates a relay device which operates in response to the output from the secondary winding 4 of the ZCT 5 while reference character I.sub.L designates a load current; reference character I.sub.0, a leakage current (zero-phase current) which flows to ground representing leakage current; reference character .PHI., magnetic flux generated in the magnetic core 1 of the ZCT 5 upon the presence of leakage current I.sub.0, and reference character V.sub.2, a secondary output voltage appearing between both terminals of the secondary winding 2, which is obtained by differentiating the magnetic flux with respect to time.
FIG. 4A is a hysteresis loop diagram showing the variation of the magnetic flux with respect to the leakage current I.sub.0. FIG. 4B is a graphical representation showing several different leakage current waveforms I.sub.0, wherein suffixes a, b and c represent small, medium and large leakage currents, respectively. FIG. 4C is a graphical representation showing the variations of magnetic flux generated by the leakage current I.sub.0 wherein, for example, a magnetic flux generated in response to the small leakage current I.sub.0a is designated by .PHI..sub.a, etc. FIG. 4D is a graphical representatation showing the variation of a secondary output voltage appearing between the two terminals of the secondary winding 2 upon the presence of a leakage current I.sub.0.
When the leakage current is small as indicated by reference character I.sub.0a in FIG. 4B, the magnetic flux generated in the magnetic core 1 in response to the leakage current I.sub.0a varies as indicated by .PHI..sub.a in FIG. 4C. As a result, the secondary output voltage V.sub.2a varies as shown in FIG. 4D. As is clear from FIG. 4D, the duration t.sub.a of the secondary output voltage V.sub.2a is sufficient to detect a leak. In contrast, in the case when the leakage current is larger as indicated by I.sub.0b or I.sub.0c (this condition is called excessive leakage), the magnetic flux .PHI..sub.b or .PHI..sub.c generated in the magnetic core 1 becomes partially saturated and therefore the secondary output voltages V.sub.2b or V.sub.2c exhibit a pulse-like waveform as shown in FIG. 4D. Accordingly, the duration t.sub.b to t.sub.c becomes smaller than that of t.sub.a.
In one prior art technique, a larger sinewave current such as I.sub.0d shown in FIG. 5A was superimposed on the actual leakage current. The resulting flux in the core then has a waveform as shown in FIG. 5B.
In this case, provided that a leakage current I.sub.0d larger than I.sub.0c, due to magnetic permeability of the atmosphere, the saturation state of the magnetic flux passing through the secondary winding is broken. As a result, a sinewave output appears at the secondary output terminals and the pulse-like output such as V.sub.2c generated when the magnetic core is magnetically saturated is superimposed on the sinewave output thereby resulting in an occurrence of output V.sub.2d as shown in FIG. 5C.
Generally, in a leakage current interrupter, a relay device which is actuated in response to the secondary output voltage V.sub.2 of the leakage current detecting element such as a ZCT is so designed that, in order to distinguish a leakage current from a noise signal, the relay device operates only when the secondary output voltage V.sub.2 is higher than a predetermined threshold voltage level V.sub.20 and further the duration of the secondary output voltage V.sub.2 is larger than a predetermined threshold duration level t.sub.0. FIG. 6 is a graphical representation showing the leakage current I.sub.0 on the horizontal axis and the duration of a secondary output voltage on the vertical axis in the case where the condition that the secondary output voltage V.sub.2 be higher than the predetermined threshold voltage level V.sub.20 is satisfied.
As is clear from FIG. 6, while the secondary output voltage V.sub.2 is higher than the predetermined threshold voltage level V.sub.20, there is a possibility that the duration t of the secondary output voltage V.sub.2 may be smaller than the predetermined threshold duration level t.sub.0, for example, in the range of the leakage current magnitude indicated by reference character E. Accordingly, even if leakage occurs, there is a possibility that the relay device 11 may not operate thereby resulting in there being no operation of the leakage current interrupter.